The present invention relates to a bipolar transistor in which switching time, especially switch off time is improved, avoiding a saturation condition by providing a vertical channel FET (field effect transistor) between base and collector.
It is said in general, that bipolar transistors have a faster switching speed compared to field effect transistors (FETs). It is true unless the bipolar transistor is driven to its saturation condition. But if the bipolar transistor is driven to its saturation state, the switching time, especially T.sub.off (the switching time to switch from an ON to an OFF state) increases.
FIG. 1 shows saturation characteristic of an ordinary emitter grounded bipolar transistor. In the figure, the ordinate shows the collector-emitter voltage V.sub.CE or base-emitter voltage V.sub.BE of a npn transistor, and the abscissa shows time. First, (t=0) V.sub.BE is low, so the transistor is cut off and V.sub.CE is high, and the base-collector junction is reversely biased. As the base voltage V.sub.BE goes up, the collector current begins to flow and the collector voltage V.sub.CE decreases by voltage drop due to the load of the collector. When the base voltage reaches V.sub.BE (on) a base current flows as a forward biased diode. The base voltage is clamped but the collector voltage further decreases until it reaches to a saturation voltage V.sub.CE (sat). At this state, the collector voltage becomes lower than the base voltage, so the base-collector voltage V.sub.BC becomes a forward bias and electrons are reversely injected from the collector to the base.
In such a situation, the electrons injected from the collector are accumulated in the base region. If the base voltage is decreased again to make the transistor OFF, the collector voltage does not respond immediately, because the electrons accumulated in the base region must be swept out. Namely, as shown in FIG. 1, when the base voltage is pulled down, the collector voltage begins to go up after a response time .DELTA.T, during which the reverse injected electrons are swept off. So, the switching off time T.sub.off of the bipolar transistor becomes longer than its switching on time T.sub.on by a factor of from ten to a hundred when it is driven to the saturation condition.
The response time .DELTA.T in the switching off operation is inherent in the bipolar transistors, so, the high speed logic circuits avoid the saturation condition of the bipolar transistors. One attempt is emitter coupled logic (ECL) which supplies a bias voltage to the base to avoid saturation. Another attempt is to clamp the collector voltage not to swing to the saturation voltage by inserting a diode, especially a Schottky barrier diode.
FIG. 2 is an equivalent circuit diagram of a bipolar transistor which is provided with a Shottky barrier diode (SBD) between base and collector electrodes. Inserting the Shottky barrier diode, the base-collector voltage V.sub.BC is clamped to a Shottky voltage V.sub.SBD determined by the Shottky barrier, and it cannot decrease further as shown by the broken line in FIG. 1. V.sub.SBD is smaller than V.sub.BC, so the collector voltage does not decrease to V.sub.CE (sat), and the reverse injection of electrons does not occur. Therefore, the collector voltage can respond quickly as shown by the broken line in the figure, and the response time .DELTA.T can be avoided.
But characteristics of the Shottky barrier diode are determined by metals used to contact with the semiconductor and the impurity concentration of the semiconductor. Therefore, in order to obtain a desirable Shottky barrier voltage V.sub.SBD for clamping the collector voltage to prevent the reverse injection of electrons as described above, the kind of metal to be used as a contact to the semiconductor is limited. Moreover, the selection of the metal is further limited from fabrication condition. Its patterning must be easy, and its Ohmic contact and adhesive property to the semiconductor must be good. Ordinarily used metals for contact with the semiconductor, such as aluminum, cannot be applied for such purpose. Therefore, the design of the device is limited in the selection of metal, and fabrication of the Shottky barrier clamping diode is critical by the present-state-of-the-art.